Silver halide developing agents are reducing agents and their solutions tend to oxidise when in contact with the air. Developer formulations, therefore, usually contain one or more antioxidants or "preservatives" which become oxidised sacrificially in order to avoid oxidation of the developing agent. Examples of such compounds are alkali metal sulphites which are contained in most developers both black and white and colour and hydroxylamine compounds which are often present in colour developers. However, even with such preservatives being present, developer solutions are still oxidised by the air in a period of weeks. It is sometimes necessary to discard a developer solution not because it is exhausted by use but because it has oxidised in the air.